Many mailpieces are today mailed using postage affixed by a postage metering system. Such a postage metering system may be either a so-called open system or a closed system. Closed systems are described in PERFORMANCE CRITERIA FOR INFORMATION-BASED INDICIA AND SECURITY ARCHITECTURE FOR CLOSED IBI POSTAGE METERING SYSTEMS (PCIB-C), Jan. 12, 1999, by the United States Postal Service. Open systems are described in PERFORMANCE CRITERIA FOR INFORMATION-BASED INDICIA AND SECURITY ARCHITECTURE FOR OPEN IBI POSTAGE EVIDENCING SYSTEMS, Feb. 23, 2000, by the United States Postal Service. Either type of such metering systems, called here simply meters, affix an indicium that includes the postage required by the mailpiece, as well as other information. In particular, the indicium on a mailpiece also includes: the postal security device (PSD) serial number, called here a meter number, of the meter used to affix the postage; the date of mailing of the mailpiece; a digital signature; an algorithm identifier for identifying the algorithm used to create the digital signature; and in case of open systems, the destination delivery point, which for destinations in the United States is simply the destination zip code.
The digital signature and destination delivery point allow the indicium on a mailpiece to be verified to a certain extent. It is possible via the digital signature and destination address to test an indicium and so to determine whether the indicium is fraudulent on its face. If an indicium survives such a test, in what is here called local verification, only two possible cases are left: either the indicium is authentic and valid or it is a duplicate (copy) of an authentic, valid indicium.
According to the prior art, in detecting indicia that are fraudulent because they are duplicates of other, valid indicia, a postal authority samples a stream of indicia, where each indicium is sampled with a random independent sample probability S. If one pair of the indicia are duplicates, then the probability of sampling both of the pair of indicia is S2. With this random independent sampling, if the sample rate is 1%, the detection probability is 0.01%, and if the sample rate is 10%, the detection probability is 1%. When the sample rate increases to 100%, of course, the detection probability also increases to 100%.
Having to sample for example 10% of all indicia to achieve a detection probability of only 1% requires what can be an extraordinary effort for so little gain.
What is needed is a system for detecting duplicates that provides greater detection probability for the same rate of sampling as is used in random independent sampling, even if the improved detection probability applies only to a subset of all indicia.